A high-purity polycrystalline silicon of a semiconductor grade is usually produced by CVD method called “Siemens method” using chlorosilanes gas mainly composed of trichlorosilane in the presence of hydrogen as a raw material (refer to, for example, Patent Literature 1: Japanese Patent Laid-Open No. 56-73617).
In general, chlorosilanes for manufacturing a polycrystalline silicon are synthesized by the reaction of a metallurgical grade silicon with hydrogen chloride (refer to, for example, Patent Literature 2: Japanese Patent Laid-Open No. 2-208217 and Patent Literature 3: Japanese Patent Laid-Open No. 9-169514) or the reduction of tetrachlorosilane by hydrogen (refer to, for example, Patent Literature 4: Japanese Patent Laid-Open No. 60-36318 and Patent Literature 5: Japanese Patent Laid-Open No. 10-29813).
However, since the synthesized chlorosilanes contain impurities derived from the metallurgical grade silicon, etc. used as a raw material, the synthesized chlorosilanes, after having been subjected to a chemical treatment for high purification (refer to, for example, Patent Literature 6: Japanese Patent Laid-Open No. 2009-62213) or a high precision distillation, is used as a raw material for producing a polycrystalline silicon.
These high purifications (i.e., the removal of impurities) are critically important especially in a polycrystalline silicon of a semiconductor grade. This is because, in case of the impurities contained being phosphorus or arsenic serving as a donor in a silicon crystal, or boron or aluminum serving as an acceptor in a silicon crystal, the impurities significantly affect the electrical properties (resistivity) of a polycrystalline silicon when incorporated thereinto even at a trace amount. Thus, the donor and acceptor impurities contained in chlorosilanes as a raw material are removed by various methods (for example, chemical treatments as described in Patent Literature 6).
Further, carbon impurities form, in a silicon crystal, an impurity level in the band gap to act as a carrier trap, or accelerate the formation of precipitation nuclei of oxygen in the crystal to induce crystal defects during the process of manufacturing a semiconductor device. Therefore, the content of carbon impurities also becomes problematic in a polycrystalline silicon of a semiconductor grade.
As one of the causes of the contamination of carbon impurities into a polycrystalline silicon, carbon-containing impurities such as alkyl chlorosilanes and hydrocarbons can be considered which are generated during the production of trichlorosilane and which are mixed into trichlorosilane or hydrogen. These carbon-containing impurities may be some times mixed into at about several tens of ppm by weight ratio when producing trichlorosilane.
In particular, methyldichlorosilane, which is the main component of methylchlorosilanes, has a boiling point (41° C.) close to that (32° C.) of trichlorosilane which is the subject of a distillation purification, and its removal is therefore difficult. For this reason, a number of methods for removing methyldichlorosilane have been proposed (for example, Patent Literatures 7-9).